1. Field of the Invention
The present invention relates to a process for producing a structural body and an etchant for silicon oxide film, and more particularly relates to a process for producing a structural body in which fine dust particles adhering to micro structural body are removed without damaging a movable element by removing a sacrificial layer made of a silicon oxide film to form an air gap and cleaning with a specific treating fluid. The process is suitable for producing a micro structural body such as micro electromechanical systems (MEMS) and semiconductor pressure sensors. The invention further relates to an etchant for silicon oxide film for use in the production of the structural body.
2. Description of the Prior Art
Recently, there have been proposed integrated semiconductor measuring devices which are manufactured, utilizing a surface-micromachining techniques, by forming sensors such as a minute pressure sensors and accelerator sensors on a silicon semiconductor substrate and mounting a circuit for processing operating signals thereof on the same silicon semiconductor substrate.
These sensors have a movable diaphragm or beam structural body, and signals are generated by the movement of the diaphragm or beam structural body. The movable diaphragm or beam structural body constituting the sensor is produced by first forming a sacrificial film and then forming a structural film as a movable element on the sacrificial film, and then removing the sacrificial film to form a diaphragm or beam structural body made of the structural film.
The production of semiconductor devices such as transistor as one of micro structural bodies generally includes a cleaning treatment with a cleaning liquid and a drying treatment to remove residual fine particles or etching residues before a process for forming a film such as a semiconductor film or after a process for etching treatment or polishing treatment. Since most of these micro structural bodies are mechanically weak, the etching of the sacrificial layer and the cleaning must be carried out with great care to avoid the damage to the micro structural body.
The micro structural body which is subjected to etching for removing the sacrificial layer and cleaning during its production, or the micro structural body to be cleaned is roughly classified into a micro structural body having a movable element and a micro structural body having no movable element. The micro structural body having a movable element is provided with a movable element supported on a stationary substrate through a narrow space, and exemplified by a micro driving body called MEMS used as parts for various sensors. The micro structural body having no movable element has a microstructure with a large aspect ratio (height/width of an opening) and is exemplified by a semiconductor substrate a fine LSI pattern and a photomask for forming fine patterns.
Construction and Production of Micro Structural Body Having Movable Element
The micro structural body having a movable element such as MEMS has been increasingly required to be made into still finer with expansion of their applications to various parts such as sensors.
The construction of the micro structural body having a movable element such as MEMS is explained by referring to FIG. 2 in which FIG. 2a is a perspective view showing the construction of a diaphragm structural body and FIG. 2b is a cross-sectional view taken along the line I-I of FIG. 2a. As shown in FIG. 2, the diaphragm structural body 40 is a pressure sensor having a diaphragm structure supported at its four sides, and includes a monocrystalline silicon substrate 42 and a diaphragm structure (bridge structure) 46 formed on the substrate 42 through an air gap 44. The diaphragm structure (bridge structure) 46 is a laminated film comprising a silicon nitride film 48 functioning as a structural film, a polycrystalline silicon film 50 functioning as a strain gauge, and a silicon nitride film 52 functioning as a protective film. When a voltage is applied between a substrate-side electrode (not shown) formed on the substrate 42 and a drive-side electrode 54 connected to the strain gauge film 50, the diaphragm structure (bridge structure) 46 functions as a movable element by moving close to or apart from the substrate 42 by an electrostatic attraction force or an electrostatic repulsion force.
Next, the process for producing the diaphragm structural body 40 is explained by referring to FIGS. 3 and 4 in which FIGS. 3a through 3d and FIGS. 4e and 4f are cross-sectional views of the intermediate products and the diaphragm structural body 40 in each production step, taken along the line I-I shown in FIG. 2a. 
As shown in FIG. 3a, a silicon oxide film 56 as a sacrificial layer is first formed on the substrate 42. Then, as shown in FIG. 3b, the silicon oxide film 56 is patterned into a desired shape. As shown in FIG. 3c, the silicon nitride film 48, the polycrystalline silicon film 50 and the silicon nitride film 52 are sequentially formed over an entire surface of the substrate 42. Then, as shown in FIG. 3d, the silicon nitride film 48, the polycrystalline silicon film 50 and the silicon nitride film 52 are etched by a reactive ion etching method to form a laminate of the silicon nitride film 48, the polycrystalline silicon film 50 and the silicon nitride film 52 having a desired shape. Then, as shown in FIG. 4e, the drive-side electrode 54 electrically connected to the polycrystalline silicon film 50 is formed. Next, as shown in FIG. 4f, the sacrificial layer made of the silicon oxide film 56 is removed by a selective etching to make the laminate formed on the sacrificial layer into the diaphragm structure 46 that is supported on the substrate 42 through the air gap 44.
Next, an example of the construction of the beam structural body is explained by referring to FIG. 5 in which FIG. 5a is a perspective view showing the construction of the beam structural body and FIG. 5b is a cross-sectional view taken along the line II-II of FIG. 5a. 
The beam structural body 60 shown in FIG. 5 is an acoustic resonator having a double-suspended beam member, which includes a monocrystalline silicon substrate 62 and a beam structure (bridge structure) 66 formed over the monocrystalline silicon substrate 62 through an air gap 64. The beam structure (bridge structure) 66 is made of a polycrystalline silicon film 68 serving as a strain gauge, i.e., a piezoelectric layer, and supported on the substrate 62 through the air gap 64.
When a voltage is applied between a substrate-side electrode 69 provided on the substrate 62 and a drive-side electrode 70 bonded to the polycrystalline silicon film 68, the beam structure 66 functions as a movable element by moving close to or apart from the substrate 62 by an electrostatic attraction force or an electrostatic repulsion force. The beam structure 66 may be either a double-suspended type or a single-supported type (cantilever type).
The MEMS having such a movable beam structural member have come to be widely used as contactors of sensors, oscillators, micro springs, optical elements, etc.
Next, the process for producing the beam structural body 60 is explained by referring to FIG. 6 in which FIGS. 6a through 6d are cross-sectional views of the intermediate products and the beam structural body in each production step, taken along the line II-II of FIG. 5a. 
As shown in FIG. 6a, a silicon oxide film 72 as a sacrificial layer is first formed on the substrate 62 and patterned to form the structures 62 shown in FIG. 5. Then, as shown in FIG. 6b, the polycrystalline silicon film 68 as a piezoelectric film is formed so as to cover the entire surface of the silicon oxide film 72. Next, the polycrystalline silicon film 68 is dry-etched and patterned into a shape of the beam structure (not shown), and then an electrode 74 is formed on the polycrystalline silicon film 68 as shown in FIG. 6c. Then, as shown in FIG. 6d, the sacrificial layer made of the silicon oxide film 72 is removed by etching. As a result, the polycrystalline silicon film 68 formed on the sacrificial layer is made into the beam structure 66 that is supported in the form of bridge over the substrate 62 through the air gap 64.
In the production of the diaphragm structural body 40, the laminate of the silicon nitride film 48, the polycrystalline silicon film 50 and the silicon nitride film 52 is patterned and then the sacrificial layer made of the silicon oxide film 56 is removed by etching. During the etching treatment, the etching gas is reacted with the material of the layer being etched to produce a reaction product that adheres to the diaphragm structure 46 as residual fine particles as shown in FIG. 7a, thereby making the production of a sensor having a desired performance difficult. Therefore, it is necessary to clean and remove the residual fine particles with a cleaning liquid.
For example, if the sacrificial layer of the silicon oxide film 56 is removed by an etching treatment with a wet etchant generally used in the production of semiconductor devices such as a hydrogen fluoride solution and a subsequent drying treatment, the diaphragm structure 46 constituted of the laminate of the silicon nitride film 48, the polycrystalline silicon film 50 and the silicon nitride film 52 which is supported on the substrate 42 through the air gap 44 tends to be damaged or stuck on the substrate 42.
The damage to the diaphragm structure 46 and its firm attachment to the substrate 42 are attributable to a suction force sucking the diaphragm structure 46 into the substrate 42 which is generated as follows. In the course of evaporation of the cleaning liquid or the rinsing liquid during the drying treatment, the liquid remaining in a very small space (air gap 44) between the diaphragm structure 46 and the substrate 42 is evaporated to reduce its volume. The reduction of volume causes the suction force between the diaphragm structure 46 and the substrate 42 by a surface tension of the liquid. If the diaphragm structure 46 is insufficient in rigidity, the diaphragm structure 46 is stuck on the substrate 42 or fractured. In addition, since the diaphragm structure 46 formed on the substrate 42 is fine and mechanically weak, it may be broken by hydraulic pressure produced by stirring of the cleaning liquid or rinsing liquid during the cleaning or rinsing step.
Similarly, in the production of the beam structural body 60, if the removal of the sacrificial layer made of the silicon oxide film 72 is conducted by an etching treatment with a wet etchant such as a hydrogen fluoride solution and a subsequent drying treatment, the beam structure 66 made of the polycrystalline silicon film 68 held over the substrate 62 through the air gap 64 tends to be damaged or stuck on the substrate 62.
Cleaning of Micro Structural Body Having No Movable Element
(1) Electron Beam Exposure Mask
Before describing the cleaning of an electron beam exposure mask, the cleaning of a resist mask or a pattern formed by using a resist mask generally employed in the patterning of semiconductor devices is described as an example of the cleaning of the micro structural body having no movable element.
In the pattern formation on a substrate employed in the production of semiconductor devices, a resist film is first formed on a patterning layer of the substrate and subjected to photo-lithographic treatment to produce a resist mask. Then, after the patterning layer is etched through the resist mask, the resist mask is removed by ashing, etc. Thereafter, the etching residues are removed by a cleaning treatment using a cleaning liquid and a rinsing treatment using pure water. Then followed by a drying treatment, the pattern is formed. Also, in the formation of the resist mask, after developed by the photo-lithographic treatment, the resist mask is cleaned with a rinsing liquid and then dried.
With the recent tendency toward increase in scale and large integration of semiconductor devices such as MOS-LSI, the LSI patterns become much finer and patterns having a line width of about 100 nm are now required. Upon forming such patterns having a line width of about 100 nm, the resist mask inevitably has an increased aspect ratio. In other words, the aspect ratio of an opening pattern for the micro structural body having no movable element becomes more and more large. The opening pattern with such a large aspect ratio causes the pattern fall as described later during a cleaning treatment, although its degree varies.
In the patterning of a line width of 100 nm or less, although the line width reaches less than the wavelength of a laser used in photolithography, the patterning by the photo-lithography is managed to be employed by suitably varying the exposure methods or masks, for example, by the use of half tone phase-shift mask. However, the patterning by the photo-lithography reaches almost its limit. Therefore, the lithography utilizing electron beam exposure has now been studied for its practical use in the patterning of semiconductor devices with a line width of 70 nm or less.
As shown in FIG. 7b, unlike the optical exposure masks, an electron beam exposure mask 80 is supported by a supporting frame 82 and constituted of a membrane 86 having opening patterns 84. The opening patterns 84 with large aspect ratios (height/width) extending through the membrane 86 are formed according to the designed circuit patterns. The electron beam reaches the resist film on a wafer through the opening pattern 84 for exposure.
When the patterns are formed through the electron beam exposure mask 80 by a reactive ion etching method, fine dust particles are attached and remain on the front and rear surfaces of the mask 80 as well as on the side wall of the opening pattern 84. Further, the fine dust particles tend to be frequently attached onto the mask during its use in the steps of transporting the mask to an exposure apparatus, fitting the mask to the exposure apparatus or exposing the mask to the electron beam. If the fine dust particles remain on the electron beam exposure mask 80 during the exposure, the fine dust particles are also imaged as a part of the pattern, resulting in the failure to obtain a pattern with a high accuracy. Therefore, the fine dust particles should be removed by cleaning with a cleaning liquid.
(2) Formation of Trench and Via for Wiring on Low-Dielectric Constant Film
To obtain a high-speed LSI, it has been inevitably required to reduce the capacity between wirings. Therefore, a low-dielectric constant (Low-k) film has come to be used as a layer insulation film between the wirings. Further, to produce an insulation film having a still lower dielectric constant, the insulation film must be made of a material having a lower-dielectric constant, and further the insulation film must have a porous structure.
In a Damascene process used for forming Cu-embedded wirings, as shown in FIG. 7(c), an etching stopper film 92 on an undercoat film 90 and a porous low-dielectric constant film 94 are etched to form a trench or via (via hole) for wiring 96 into which a wiring material, e.g., Cu is embedded and then polished to form a Cu-embedded wiring (not shown). After forming the trench or via for wiring 96 by etching the etching stopper film 92 and the porous low-dielectric constant film 94, as shown in FIG. 7(c), fine dust particles resulted from the reaction between the etching gas and the porous low-dielectric constant film 94 are attached onto the side walls of the trench or via for wiring 96 as well as the surface of the porous low-dielectric constant film 94. To produce embedded wirings successfully, the remaining fine dust particles should be removed with a cleaning liquid.
As described above, in the production of semiconductor devices, the fine dust particles should be removed with a cleaning liquid. However, the cleaning liquids include those suitable and unsuitable for removing the fine dust particles as described below.
Generally, water is widely used as the cleaning liquid for removing the fine dust particles by a wet cleaning. However, water fails to reach the bottom of the trench or via for wiring having a large aspect ratio because of its high surface tension. Even if reaching the bottom, it is difficult to remove the etching liquid remaining after the etching treatment from the trench or via for wiring, thereby failing to dry the trench or via.
Another significant problem caused upon drying the trench or via for wiring with fine patterns is a pattern fall. The pattern fall occurs upon drying the cleaning liquid or the rinsing liquid, and becomes more remarkable for patterns having a larger aspect ratio. The pattern fall is a phenomenon that a pattern is broken by a bending stress (surface tension or capillary force) which is generated during the drying after the cleaning by the pressure difference between the outside atmosphere and the cleaning or rinsing liquid remaining in patterns such as the trench or via for wiring. The capillary force varies depending on the surface tension of the cleaning or rinsing liquid which is generated at a vapor-liquid interface between the patterns and distorts the patterns formed. Therefore, the surface tension of the cleaning or rinsing liquid is an important factor for selecting a suitable cleaning or rinsing liquid.
In the wet cleaning of the porous low-dielectric constant film, the pores tend to be collapsed by the pressure difference due to the formation of a vapor-liquid interface during the cleaning liquid such as water goes in or out of the pores, posing a problem of increasing the dielectric constant.
Drying by Supercritical Fluid
As described above, in both cases of manufacturing the micro structural body having a movable element (in particular, in the step of etching the sacrificial layer) and cleaning the micro structural body having no movable element, the degree of the surface tension of the cleaning liquid has a large influence on the occurrence of damage to the micro structural body.
It is expected that the damage by the surface tension may be prevented by performing the cleaning and drying by using a fluid having a surface tension lower than that of water (about 72 dyn/cm), for example, using methanol (about 23 dyn/cm). The attachment of the movable element onto the substrate and the fracture of the patterns can be prevented by the drying after replacing water with methanol as compared to the drying of water. However, since the surface tension of methanol is still higher, the problems of the fracture of patterns and the pattern fall cannot be effectively solved.
The problems such as pattern fall due to the surface tension can be solved by using a fluid having a surface tension of zero as the cleaning or rinsing liquid or by drying after replacing a common rinsing liquid with a fluid having a surface tension of zero. The fluid having a surface tension of zero is a fluid in a supercritical state, i.e., a supercritical fluid. The supercritical state is one of phases taken by a substance in a state above the temperature and pressure specific to the substance, i.e., the critical temperature and the critical pressure. A substance in its supercritical state has unique properties that the viscosity is considerably low and the diffusion coefficient is extremely large despite its dissolving power to other liquids and solids similar to that of the substance in its liquid state, namely, the supercritical fluid may be a liquid having properties of gas. The supercritical fluid does not form a vapor-liquid interface to show a surface tension of zero. Therefore, if the drying is conducted in the supercritical state showing no surface tension, the pattern fall can be completely prevented.
The supercritical fluid is rapidly gasified by reducing the pressure of surrounding atmosphere to the critical pressure or lower. Therefore, the drying of the supercritical fluid after the cleaning treatment can be done by gasifying it under reduced pressure after releasing the supercritical fluid. Thus, the drying after cleaning with the supercritical fluid is easily completed.
The cleaning using the supercritical fluid may be conducted as follows. After separating the movable element partially or entirely apart from the supporting substrate by etching, or after forming micro patterns having a large aspect ratio by etching, the resultant product as-etched or after cleaning with a cleaning liquid or replacing with another liquid is brought into contact with the supercritical fluid stored in a pressure container. By such a contact, the remaining etchant, cleaning liquid and another liquid are dissolved into the supercritical fluid and removed together with etching residues.
Successively, the supercritical fluid is gasified and discharged by reducing the inner pressure of the pressure container to the critical pressure or lower while maintaining the pressure container at the critical temperature or higher, and thereafter, the micro structural body was taken into the outside atmosphere. Since the surface tension of the supercritical fluid is extremely small, the stress due to the surface tension applied onto the micro structural body during the removal of the supercritical fluid from the surface of the micro structural body is negligibly small. Therefore, by the use of the supercritical fluid as the cleaning liquid, the cleaning liquid, etc. adhering to the micro structural body during the etching treatment may be effectively removed without causing the deformation of damage of the micro structural body.
There has been proposed a method of introducing a supercritical fluid into a reaction chamber while or after removing water present inside of the chamber to dry the material immersed in a liquid (JP 2000-91180 A, page 4). There have been also proposed a method and an apparatus in which a liquid attached to a micro structural body is removed by dissolving in a supercritical fluid in a pressure container, the supercritical fluid is gasified for removal by reducing the inner pressure of the container to the critical pressure or lower, and then the dried micro structural body is taken into the outside atmosphere (JP 9-139374 A, page 5).
In the production of the diaphragm structural body or beam structural body, to prevent the adhesion of the diaphragm or beam member to the substrate during the drying by evaporating the etchant or rinsing liquid after removing the sacrificial layer made of the silicon oxide film by wet-etching, the drying is made by a supercritical drying. In the supercritical drying, the etchant (aqueous solution) used for the wet-etching must be replaced by a supercritical carbon dioxide fluid without exposing the structural body to the surrounding atmosphere.
However, since the aqueous etchant is immiscible with the supercritical carbon dioxide fluid, a complicated treatment is required in which the etchant is first replaced by a third solvent such as alcohol and then the alcohol is replaced by the supercritical carbon dioxide fluid, or the etchant is first replaced by the third solvent and then the third solvent is replaced by the supercritical carbon dioxide fluid. Further, since the replacement with the third solvent should be conducted without exposing the material to be cleaned, i.e., the micro structural body to the surrounding atmosphere to avoid the generation of surface tension, the solvent is consumed unfavorably in a large amount.
In addition, since the electrodes for the diaphragm structural body or beam structural body are generally made of a conductive metal material such as aluminum and aluminum alloys, the electrodes are corroded upon the exposure to the liquid etchant for removal of the sacrificial layer despite the use of the supercritical fluid for drying.
In the cleaning for removing only fine particles from the micro structural body, since a vapor-liquid interface is formed during the immersion in an aqueous solution for wet cleaning, the adhesion of the diaphragm or beam member to the substrate occurs during the cleaning. To avoid this problem, it is required to immerse the micro structural body in the supercritical fluid to create the supercritical state, replace by an alcohol, and then replace by the aqueous solution for cleaning. In addition, since the etchant used for wet etching should be replaced by the supercritical carbon dioxide fluid for drying without exposing the micro structural body to the surrounding atmosphere, the solvent is consumed unfavorably in a large amount.
The supercritical carbon dioxide fluid has dissolving properties similar to those of non-polar organic solvents, and therefore, shows a dissolving selectivity when used alone. Namely, the supercritical carbon dioxide fluid is effective for removing low-molecular organic substances, fats, oils and waxes, but ineffective for removing fine dust particles made of mixed inorganic compounds, fibers or organic high-molecular compounds such as plastics. Therefore, the single use of the supercritical carbon dioxide fluid is unsatisfactory for removing the silicon oxide film as the sacrificial layer and etching the silicon oxide film necessary for the removal of the particles. Therefore, there has been made study on the etching of the silicon oxide film under a supercritical state by adding an additive effective for the etching of the sacrificial film and removal of particles, such as a fluorine compound capable of etching the silicon oxide film, into the supercritical carbon dioxide fluid.
For example, there have been proposed a method of removing a silicon oxide film simultaneously with the removal of contaminants by using a supercritical fluid containing a fluorine compound or the fluorine compound and an organic solvent as a dissolving aid for contaminants (JP 64-45125 A, JP 10-135170 A, JP 2003-513342 A, and JP 2003-224099 A), and a method of etching an interlayer film using a supercritical fluid containing a fluorine compound for forming a hollow wiring (JP 2002-231806 A). However, the chemical substances such as fluorine compounds capable of etching the silicon oxide film are generally soluble in solvents such as water, but hardly soluble in the supercritical carbon dioxide fluid and have a low etching rate. In particular, in the formation of a diaphragm member or a beam member of sensor parts, the silicon oxide film as a sacrificial layer having a thickness of several tens to several hundreds nanometers must be completely etched, and the single-wafer cleaning should be completed within several seconds to one minute. However, the proposed methods fail to obtain a practical etching rate for etching the silicon oxide film.